Just like you get on your fridge freezer or washing machine, the coloured arrows show you just how efficient your window is. Of course there are scales to those labels too, so look out for the Energy Index to tell you just how well the window performs. The newest labels have the new A+ band for windows (Purple) that achieve an A+10 or greater.
The energy efficiency of a window or door is calculated using two different factors; G Value & U Value.
G VALUE is the calculation of solar gain made by your window or door. Simply put how much energy (heat) is generated by your window.
U Value is the calculation of heat loss through your window or door.
The two combined create the Energy Index. Windows and doors with a low U Value and high solar gain will get a better Energy Index and be the best choice for new windows and doors.